In recent years, in proportion to the rapid increase of the transmission speed of communication between LSIs or substrates, a serial transmission method for superimposing data on a clock in a signal line and transmitting the data is prevailing in place of a conventional parallel transmission method using a tandem clock. In the serial transmission method, it is necessary to dramatically increase the transmission speed per one signal line and, in association with that, the attenuation of the high frequency component of transmission signals caused by the skin effect of a transmission line conductor, the dielectric loss of an insulation material, or the like increases and, on the receiver side, deterioration of signal quality depending on a transmission code sequence called ISI (Inter-Symbol Interference) appears in an input waveform. As a result, an eye pattern on the receiver side narrows in both the time-axis (width) direction and the amplitude (height) direction and that causes a reception error to occur.
FIG. 5 comprises graphs conceptually showing that signals deteriorate by ISI. As shown in FIG. 5A, when a code sequence is a pattern wherein “H” levels and “L” levels appear alternately, both the time and the amplitude of a received signal are well aligned. As shown in FIG. 5B however, in a code sequence wherein an “H” level code appears after two “L” level codes appear consecutively, the deviation of received signals in the time-axis direction, namely pattern jitters, appears and also an amplitude may not reach a prescribed level in some cases. The deviation in the time-axis direction (pattern jitters) 501 and the deviation in the amplitude direction 502 appear as the reduction of the width and the height of an eye pattern on the receiver side, respectively.
The reduction of the height of an eye pattern, namely the deviation in the amplitude direction, can be compensated to some extent by amplifying signals with an amplifier. However, there is no means for simply compensating the reduction of the width of an eye pattern, namely pattern jitters, because it is impossible to extend the time axis. In addition, although to take out a clock from a signal waveform itself with a clock data regenerator (CDR) is generally employed in a recent serial transmission method, the jitters of received signals: largely deteriorate the performance of the clock data regenerator; and cause a reception error caused by synchronization deviation to appear. Therefore, how to suppress or compensate pattern jitters on a receiver side is a big problem in the event of high speed signal transmission.
In order to solve the problem, employed is a method for expanding an eye at a receiving end by transmitting a waveform distorted beforehand from a sender side (sender side equalization) in consideration of the ISI of a transmission line. As methods for the sender side equalization, there are roughly two methods; a method of changing a pulse amplitude in accordance with a code sequence to be sent (sender side pulse amplitude equalization) and a method of changing a pulse time span in accordance with a code sequence to be sent (sender side pulse width equalization).
The sender side pulse amplitude equalization: can determine the variation of amplitude by linear computation (an FIR filter) of a transmission data sequence; hence can be realized comparatively easily; and is widely used currently. In the sender side pulse amplitude equalization however, the equalization is carried out at each symbol and hence the ISI faster than a symbol rate cannot be compensated in principle, and consequently the effect of ISI reduction is limited.
In contrast, in the case of the sender side pulse width equalization, in principle it is possible to reduce pattern jitters to zero by optimally controlling the variation of a pulse width. Further, it is possible: to increase the average level of a transmission waveform to the limit of the capability of a transmitter; and hence to increase also the height of an eye pattern on the receiver side.
As an example of applying the sender side pulse width equalization to communication between LSIs, there is a transmission signal correction circuit disclosed in JP-A No. 2005-057686. FIG. 2 is a block diagram showing a transmission signal correction circuit disclosed in JP-A No. 2005-057686. In FIG. 2, a data row inspection circuit 203: inspects a code sequence 201 sent from an internal circuit to be sent outside; and determines an optimum pulse width. An M-stage variable delay circuit 202 delays the code sequence sent from the internal circuit on the basis of the pulse width, and then a transmission driver circuit 204 amplifies signals to a transmission level and outputs the signals.
In the transmission signal correction circuit of this configuration, the internal signals are delayed and output so as to: compensate pattern jitters beforehand on the receiver side; and get the optimum pulse width determined by some method in accordance with the transmission code sequence. As a result, the influence of pattern jitters is negated and a beautiful observation waveform can be obtained on the receiver side.
A big problem in the sender side pulse width equalization is that an optimum pulse width responding to a code sequence is hardly determined since the transmission code sequence and the optimum pulse width are in a complicated nonlinear relation. As a method for solving the problem, it is considered to make a table to store an optimum pulse width wherein a transmission code sequence is used as a search key in advance of actual transmission.
In a digital signal write/read device such as an optical disk or a magnetic disk for example, carried out are the processes of: writing a plurality of specific code sequences on trial in advance of actual data writing; and deciding and storing in a table an optimum pulse width responding to a code sequence from the observation result of the edge positions of the reproduced signals.
FIG. 3 is a simplified block diagram showing an optical disk memory unit disclosed in JP-A No. 11(1999)-259863. Every time when a recording medium 301 is exchanged, a plurality of specific code sequences are written on trial in the region for trial writing on the recording medium in advance of the start of the writing of actual data. On this occasion, a controller stops a pulse width adjustment circuit 305 and the pulse width of writing is kept constant. After the finish of the trial writing, the data written on trial are read with a pickup 302 and amplified with an amplifier 306, and the edge position of the data is detected with an edge timing detector 308. The magnitude of the deviation of the edge position of the data written on trial from its proper position is judged with a judging device for pulse width setting, a pulse width adjustment amount required for moving the edge position to its proper position is computed with a data conversion circuit 310, and the computed result is set in a pulse width adjustment amount table. The above processes are those carried out in advance of the writing of actual data every time when a recording medium is changed.
When actual data are written, the pulse width adjustment amount table 311 is looked up by using a code sequence to be written as a search key and an optimum pulse width is decided. After a pulse time span is adjusted so as to be the optimum pulse width with the pulse width adjustment circuit 304, the data are written into the recording medium 301 through a laser driver 303 and the pickup 302.
With an optical disk memory unit disclosed in JP-A No. 11(1999)-259863, it is possible to obtain a beautiful waveform having fewer jitters at reading by determining an optimum pulse width adjustment amount in response to a code sequence by trial writing every time when a recording medium is changed.
Here, estimated is the size of a table required for realizing such a configuration of storing an optimum pulse width responding to a code sequence in the table as the optical disk memory unit disclosed in JP-A No. 11(1999)-259863. ISI decreases as the distance from a target symbol increases. Therefore, in the event of equalization, only the code sequence of the length N that cannot ignore the influence of ISI may be taken into consideration. Here, N is a constant determined by the state of a line and the sensitivity of a receiver. In the optical disk memory unit disclosed in JP-A No. 11(1999)-259863, it is necessary to store an optimum pulse width in a table for each of the code sequences of the length N. Consequently, a table size proportional to 2 to the N-th power is necessary.